This invention relates generally to the field of abandonment of offshore structures. More specifically, the present invention is a method for disposing of a large steel jacket structure in one piece by suspending the jacket under a tow vessel, towing the jacket to a disposal site, and safely and quickly releasing the jacket at the disposal site.
When an offshore oil or gas reservoir is depleted and is no longer economical to produce, an alternative use must be found for the platform or it must be decommissioned. Offshore platform decommissioning will become an increasing problem for the international oil and gas industry as many of the early fields are coming toward the end of their lives. There are currently about 7000 offshore platforms in existence worldwide. These platforms range from small wellhead structures weighing only tons or at most a few hundred tons to large multi-well steel or concrete structures weighing tens of thousands of tons. They are located in water depths ranging from only a few meters to about 150 meters, with a small number in depths around 300 meters.
Many offshore platforms have already been abandoned. To date most have been relatively small platforms located in shallow water. With smaller platforms, the piles can be cut below the mudline and the jacket can be lifted in one piece and either returned to shore for scrapping, returned to shore for refurbishment and reuse, or transported to a shallow water artificial reef site. However with platforms located in deeper water, the problem of how to remove, transport, and dispose of the larger substructure jacket becomes more difficult and the associated costs increase.
There are several options for decommissioning an offshore platform at the end of a producing field""s life including: (1) leaving the platform in-situ (e.g., artificial reef or abandoned with suitable navigational markings); (2) partial removal of the platform; and (3) complete removal of the platform. The technical feasibility of the option used will depend on physical properties specific to each platform including: water depth, location, physical condition, size, weight, buoyancy, structural configuration and structural integrity. As water depth and jacket sizes increase the major cost components of removal of large steel or concrete structures increase because of high barge and crane costs, time spent offshore, and abandonment of equipment.
It is almost always possible to cut the steel jacket into sections which are small enough to be handled with the available offshore lift vessels. However, this may be a very expensive solution. In order to cut a large steel structure into pieces that are liftable, many large diameter jacket legs and diagonal braces must be cut underwater. This cutting can be done for example with explosives, divers using torches or abrasive cutters. The major drawbacks to using this method are that it is time consuming and the multiple underwater cutting required considerably increases the risks to divers and equipment. For very large structures, the time and costs associated with piecemeal removal go up due to the increase in the number of pieces to be removed. In very deep water, the costs could be significantly impacted due to the increased problems with working at depth. Also, because the rental rates for the support equipment such as derrick barges and dive support vessels are quite high, these long duration cutting activities greatly increase the cost of removal.
Assuming that disposing of the jacket in-situ will be a viable disposal option for only a limited number of platforms, the least expensive option for many others may be to refloat the jacket in one piece by means of internal and auxiliary buoyancy and then to tow the structure to deep water for disposal. The primary ways to provide the required auxiliary buoyancy are to lift the jacket with a derrick barge or attach some form of auxiliary buoyancy device to the jacket. This auxiliary buoyancy device could be a steel tank, a steel buoyancy tube installed in a skirt pile guide, an inflatable rubber lift bag, or a vessel such as a barge or boat. The methods involving auxiliary buoyancy tanks are usually expensive because the tanks are very large and will likely not be reusable on other projects. When a derrick barge or other rented vessel is used to provide the needed auxiliary buoyancy; there is also the problem of how to release the jacket from the vessel. With a derrick barge, it is not possible to design a quick release of the vessel load from the crane hook because of the danger that the rebound of the crane boom would cause the boom to buckle and fail. If the jacket is attached to a barge or other vessel, the problem of release still exists: because the attachment has to be strong enough to hold the structure during the tow, it becomes difficult to devise a quick release method that will reliably and quickly release such large connections.
While current methods of jacket removal may be adequate for removal of smaller jackets, the cost of removal is expected to rise dramatically in the future with more larger structures reaching the end of their operational lives. These circumstances necessitate a simpler and more cost-effective method of conducting platform removal operations. Therefore, it would be desirable to have a method for transporting and disposing of a jacket which allows for a safe and quick release of the jacket. The present invention satisfies this need.
The present invention is a method of transporting an offshore platform substructure, commonly known as a xe2x80x9cjacketxe2x80x9d, from a first offshore location to a second offshore location for release at the second location. The method uses a floating vessel, such as a barge, that has at least two release means attached to corresponding first and second ends of the vessel. Each of the release means is adapted to pivot about a pivot point located outboard of the vessel. A first lift rigging means is used to lift the first end of the jacket until the first lift rigging means engages with the first release means at a position outboard from but proximate to the pivot point of the first release means. A second lift rigging means is attached to a second end of the jacket, and the second-end of the jacket is lifted until the second lift rigging means engages with the second release means at a position outboard from but proximate to the pivot point of the second release means. Thus the jacket is suspended substantially under the vessel. The vessel is then moved to the second offshore location, and the first and second release means are activated, which allows each of the release means to pivot about their respective pivot points and release the jacket.
Each of the lift rigging means can be comprised of a spreader bar having at least one skid shoe attached thereto. At least two padeyes are attached at opposite ends of the spreader bar and a lift sling is attached to each of the padeyes. At least two jacket support slings are attached to the spreader bar. Each of the release means can be comprised of one rocker beam having a release-end and a connector located at the release-end. The connector is used to attach the rocker beam to the vessel, and the rocker beam is adapted to pivot about the pivot point upon disengaging the connector.